A typical thin-film thermal printhead, disclosed in e.g. the following Patent Document 1, has a structure as shown in FIGS. 3 and 4 of the present application. The thin-film thermal printhead B shown in the figures has a lamination structure including an insulating substrate 101, a heat-retaining glaze layer 102 formed on the insulating substrate, a resistor layer 103 formed as a thin film on the heat-retaining glaze layer 102 by e.g. sputtering, a conductor layer 104 formed similarly as a thin film on the resistor layer 103, and a protective film 105 covering the resistor layer 103 and the conductor layer 104. In the example shown in FIG. 4, the heat-retaining glaze layer 102 includes a gently bulging portion 102c. The resistor layer 103 extends continuously from one base to the opposite base of the bulging portion 102c over the top of the bulging portion, but is divided at regular intervals in the longitudinal direction of the bulging portion 102c (FIG. 3). The conductor layer 104 is partially removed at the top of the bulging portion 102c. The conductor layer includes a plurality of individual electrodes 104a extending from the bulging portion 102c in one direction and electrically connected to the output pad of a non-illustrated driver IC, and a common electrode 104b provided with a plurality of comb-teeth 104c extending from the bulging portion 102c in the opposite direction from the individual electrodes 104a. 
When voltage is applied between each of the individual electrodes 104a and the common electrode 104b, current flows through the portions 103c (heating dots) of the resistor layer 103 which are located on the top of the bulging portion 102c to generate Joule heat. The heating dots 103c are pressed against a printing medium via the protective film 105, whereby thermosensitive printing is performed.
The protective layer 105 may be formed using a hard material such as SiO2 by a thin film formation technique such as sputtering to have a thickness of not more than about 5 μm, for example. The protective layer 105 is a portion to rub against the printing medium such as thermal recording paper or an ink ribbon in printing, and hence, needs to be abrasion-resistant. Further, the protective layer 105 serves to prevent the corrosion of the resistor layer 103 or the conductor layer 104 by preventing moisture contained in the atmosphere or Cl− or Na+ ions or the like contained in the printing medium from coming into contact with these layers.
However, in the case where the thickness of the protective layer 105 is not more than 5 μm, when foreign matter such as dust in the printer enters the space between the thermal printhead B and the printing medium (not shown), the protective layer 105 is peeled off by the foreign matter to partially expose the resistor layer 103 or the conductor layer 104. In this case, the resistance of the resistor layer 103 is largely changed due to oxidation or corrosion, whereby the print quality is considerably deteriorated. Further, in forming the protective layer 105 by sputtering, film formation defects such as a crack starting from the stepped portion 104d between the resistor layer 103 and the conductor layer 104 or a pinhole caused by foreign matter adhering to the resistor layer 103 or the conductor layer 104 are likely to occur. As a result, the Cl− or Na+ ions or the like infiltrate to corrode the resistor layer 103 and the conductor layer 104, so that the resistance of the conductor layer 103 is largely changed in a relatively short period of time.
Patent Document 1: JP-A-H08-207335
A method to solve the above-described problems is to form the protective layer by bias sputtering. By employing the bias sputtering, a protective layer having few film formation defects and a high sealing performance can be obtained. However, the protective layer 105 formed by bias sputtering has a large stress therein, and hence, is likely to peel off from the conductor layer 104 due to the friction with the printing medium.